NAIROBI, Kenya (AP) — The Islamic extremist group that killed scores of people at a Nairobi mall has now attacked two Kenyan towns near the Somali border, killing three people. The leader of the Somali group affiliated with al-Qaeda said the attacks will continue until Kenyan troops are withdrawn from Somalia.

The leader of al-Shabab said in a message that there is no way Kenya can "withstand a war of attrition inside your own country."

"Make your choice today and withdraw all your forces," said Ahmed Abdi Mohamed Godane, who goes by his nom de guerre Mukhtar Abu Zubayr, in a new statement posted on the Internet late Wednesday. "Otherwise be prepared for an abundance of blood that will be spilt in your country, economic downfall and displacement."

Al-Shabab said the Nairobi mall attack was not only directed at Kenya, but was also "a retribution against the Western states that supported the Kenyan invasion and are spilling the blood of innocent Muslims in order to pave the way for their mineral companies," according to the statement from Godane.

Al-Shabab attacked Nairobi's upscale Westgate mall Saturday and held it for four days in a siege in which at least 67 people were killed. Forensic experts from around the world, including the U.S., Britain, Germany and Canada, continued their work Thursday reconstructing events in the crime scene including by carrying out fingerprint, DNA and ballistic analysis.

Wednesday night, al-Shabab attacked the border town of Wajir, 240 miles southwest of Mandera. One person was killed and four wounded after a gunman opened fire and threw grenades.

Kenya has suffered many such attacks by al-Shabab along its 423 mile border with Somalia but they take on new significance following the Westgate Mall attack.

At the city morgue in central Nairobi where bodies recovered from the mall have been taken, families wept as they waited to collect the bodies of their loved ones, and several expressed frustration about how long the process was taking.

Peter Mwalaia said he had been waiting to collect the body of his relative since Wednesday, while others had been waiting as long as Saturday.

"We have been waiting for the body ... to transfer it to a mortuary that is closer to our place but we have not been allowed," he said. "We have been told that the body has not been cleared by the government."

A person photographs the collapsed upper parking garage at the Westgate mall on Oct. 1 in Nairobi, Kenya. Gunmen took over the mall on Sept. 21 and killed 67 people. (Photo: Jason Straziuso, AP)

A Muslim woman offer prayers during a service organized by the Asian Muslim community in Kenya for the victims of the Westgate shopping mall terrorist attack in front of the mall building on Sept. 29 in Nairobi. (Photo: Dai Kurokawa, epa)

A photograph released on Sept. 26 shows several floors of a parking garage that collapsed at the Westgate Shopping Mall in Nairobi, Kenya, during a four-day battle with Somali militants. At least 67 people were killed during the attack that began Sept. 21. (Photo: Office of the Kenyan President via AP)

Alyaz Merali, a Muslim boy who was shot twice when militants attacked the Westgate Shopping Mall, attends a funeral procession Sept. 25 for his mother and sister who were killed in the attack in Nairobi, Kenya. (Photo: Uriel Sinai, Getty Images)

Ramesh Vaya, right, lights the funeral pyre of his wife, Malti, who was killed in the attack, at the Hindu Crematorium on Sept. 24 in Nairobi. Ramesh and his brother both lost their wives in the attack. (Photo: Kate Holt, AP)

Kenyan soldiers clear the top floor of the Westgate mall. Somali militants, claiming they were still holding hostages, battled Kenyan troops on the fourth day of a bloody siege at the mall. (Photo: Carl De Souza, AFP/Getty Images)

A riot police officer clears an area near the Westgate Shopping Mall in Nairobi. Kenya's military launched a major operation against al-Shabab rebels in the mall and said it had rescued most of the hostages. (Photo: James Quest, AFP/Getty Images)

Smoke rises from a building as a government assault team conducts a hostage rescue operation at the Westgate mall. Kenyan troops are continuing an operation to free hostages after militants from the Somali Islamist group al-Shabab entered the mall on Sept. 21 and opened fire on shoppers. (Photo: Dai Kurokawa, epa)

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The Red Cross had set up a special information tent on the lawn of the morgue to help answer peoples' questions, but people say they are still largely in the dark.

"We don't know exactly what is happening," said Julius Zeirok, the friend of a victim. "We are outside waiting to be served."

Experts from the U.S., Germany, and the international police agency Interpol were seen entering and leaving the facility, along with local workers in white coveralls and rubber boots.

Two British pathologists were working with their Kenyan counterparts inside performing autopsies, a senior morgue official said on condition of anonymity because he was not authorized to speak with the press.

Bullets and shrapnel being removed from the corpses are being turned over to police as evidence, chief government pathologist Johansen Oduor told the AP.

"A lot of them died from bullet wounds — the body, the head, all over," he said. "Some also died from grenades, shrapnel."

He refused to say how many bodies were in the morgue but did say that he was told to expect more — though would not say how many.

The Kenyan Red Cross said Thursday that 61 people remain missing and many worry that they may be buried under the rubble in the mall — though the government has said they do not believe there are many more victims inside. The Red Cross earlier said 71 people were missing, but that number has reduced as bodies in the morgue have been identified.

Several FBI agents from New York City, including members of the Joint Terrorism Task Force, bomb squad technicians and evidence recovery specialists have been sent to Nairobi, a U.S. law enforcement official confirmed. The official spoke on condition of anonymity because he wasn't authorized to discuss the operation.

The international investigation is being coordinated by Interpol, which sent an incident response team that arrived in Nairobi on Wednesday, the Kenyan government said.

The Interpol team includes disaster victim identification and data specialists who will carry out real-time comparisons of evidence collected inside the mall against the France-based agency's database on DNA and fingerprints from its 190 member country network, said Interpol official Jean-Michel Louboutin.

"Whether it be through comparison of information against Interpol's global databases, or the issuance of a notice to identify a victim, locate a wanted person, or seek additional information about suspects, we will offer all necessary assistance to help bring those responsible to justice," Louboutin said in a statement.

He added that Interpol is also ready to mobilize additional support if needed, including from its counter-terrorism and criminal analysis units.

There have been no details on what the international team has found so far in the bullet-scarred, scorched mall but their work is expected to take a week, said Kenyan police spokeswoman Gatiria Mboroki.

Washington is providing technical support and equipment to Kenyan security forces and medical responders, said U.S. Ambassador Robert Godec. The U.S. is assisting the investigation to bring the attack's organizers and perpetrators to justice, he said Wednesday.

At least 18 foreigners were among those killed when the militants entered the Westgate Mall on Saturday, firing assault rifles and throwing grenades, including six Britons and citizens from France, Canada, the Netherlands, Australia, Peru, India, Ghana, South Africa and China.

In addition to those killed at the mall, another 175 people were injured, including more than 60 who remain hospitalized.

Authorities have said at least five al-Shabab attackers were killed and another 11 suspects have been taken into custody.

In his statement, the al-Shabab leader Godane said only that "some" of his fighters had been killed, possibly suggesting that others escaped.

During the four-day fight at the mall, the building's roof collapsed, causing massive destruction. The collapse came Monday, shortly after four large explosions rang out followed by billows of black smoke. A government minister said the terrorists had set mattresses on fire, causing the roof to collapse, but it seemed unlikely the fire would have caused the massive destruction.

Al-Shabab, whose name means "The Youth" in Arabic, first began threatening Kenya with a major terror attack in late 2011, after Kenya sent troops into Somalia following a spate of kidnappings of Westerners inside Kenya.

The mall attack was the deadliest terrorist attack in Kenya since the 1998 al-Qaeda truck bombing of the U.S. Embassy in Nairobi, which killed more than 200 people.